There are optical disc devices that reproduce information by emitting an optical beam toward an optical disc, such as CD (Compact Disc), DVD (Digital Versatile Disc), or Blu-ray Disc (Registered Trademark: hereinafter, referred to as BD), and detecting a reflected beam. Such optical disc devices have come into wide use.
In such an optical disc device, an optical beam is emitted to the optical disc so as to change reflectance of the optical disc locally or the like, thereby recording information.
With regard to the optical disc, the size of a beam spot formed when the optical beam is condensed by an objective lens or the like is approximately determined based on λ/NA (λ: the wavelength of the optical beam, NA: numerical aperture). The resolution is proportional to that value. For example, a BD-type optical disc of 120 mm in diameter can record about 25 GB of data per layer.
Meanwhile, the optical disc is designed to record various kinds of information, such as various kinds of content including music or video content, various kinds of data for a computer, and the like. In recent years, the amount of information has been growing due to high-definition video data or high-quality music data, and the number of pieces of content to be recorded on one optical disc has been increasing. For this reason, the optical disc is required to have larger capacity.
To simplify the structure and increase the capacity of the optical disc, some optical disc devices make use of holograms so as to perform standing-wave recording on a flat recording layer of an optical disc in a multilayer structure (for example, see Patent Citation 1).    Patent Citation 1: JP-A-2007-220206 (FIG. 24)
With regard to the optical discs that correspond to the optical disc device configured as above, since the recording layer is flat, the optical disc is also provided with a reference layer on which tracks or the like are formed for positioning. The reference layer allows the optical disc device to specify recording positions inside the recording layer.
For example, as shown in FIG. 1, inside a recording layer 101 of an optical disc 100, there are a plurality of layers (hereinafter, referred to as mark layers Y) having recording marks spirally arranged on a predetermined plane. In the optical disc 100, a predetermined distance DG refers to a distance between a target track TG of the reference layer 102 and a target mark layer YG of the recording layer 101 in terms of a direction along a normal line XD of the reference layer 102.
When reproducing information, the optical disc device 1 lets a predetermined reference optical beam LS pass through a beam splitter 3. An objective lens 4 focuses the beam on the reference layer 102 of the optical disc 100.
The optical disc device 1 detects a reflected reference optical beam after the reference optical beam LS is reflected by the reference layer 102 of the optical disc 100, and performs focus control and tracking control of the objective lens 4 in accordance with the detection result, thereby focusing the reference optical beam LS on the target track TG of the reference layer 102.
The optical disc device 1 lets the beam splitter 3 reflect an information optical beam LM. The objective lens 4 whose position is under control focuses the beam on the target mark layer YG from among mark layers Y formed inside the recording layer 101.
In this case, on the assumption that an optical axis XL of the reference optical beam LS and the information optical beam LM is aligned with the normal line XD of the optical disc 100, the optical disc device 1 makes sure that the distance between a focal point FS of the reference optical beam LS and a focal point FM of the information optical beam LM is DG on the optical axis XL. As a result, the focal point Fb of the information optical beam LM can be focused on the target mark layer YG.
Incidentally, after the information optical beam LM is reflected by each recording mark of the target mark layer YG, the optical disc device 1 detects a reflected information optical beam, thereby reproducing recorded information.
However, there is a possibility that the optical disc 100 inclines with respect to the optical disc device 1 due to the deformation of the optical disc 100, so-called surface wobbling, or the like.
For example, as shown in FIG. 2, which shows the same thing as FIG. 1, if the optical disc 100 tilts at θ degrees, the distance between the reference layer 102 and the target mark layer YG on the optical axis XL becomes (1/cos θ) times longer than the distance DG, so the distance differs from the distance DG.
In this case, even if the reference optical beam LS is focused on the reference layer 102 of the optical disc 100, the focal point FM of the information optical beam LM may not be on the target mark layer YG. Accordingly, information may not be read from the target mark layer YG.
That is, if the reference optical beam LS is merely focused on the reference layer 102, this leads to the failure of focus control: the optical disc device 1 may not be able to put the focal point FM of the information optical beam LM on the target mark layer YG. This may result in a significant decrease in accuracy of information reproduction.
The invention has been finalized in consideration of the above points and is intended to suggest an optical disc device and a focus control method that can improve accuracy in reproducing information from an optical disc.
In order to solve such problems, there is provided a focus control method according to the invention. The focus control method includes the steps of emitting a reference optical beam so as to irradiate a reference layer provided in an optical disc, emitting an information optical beam so as to reproduce information from a mark layer on which recording marks representing the information are arranged inside a recording layer provided in the optical disc at a predetermined distance from the reference layer, shaping the information optical beam entering an objective lens such that, in terms of a direction of an optical axis of the information optical beam, a distance between the focal points of the reference optical beam and the information optical beam focused by the objective lens becomes equal to a predetermined distance, moving the objective lens in the direction of the optical disc of the information optical beam, receiving a reflected reference optical beam which is the reference optical beam reflected by the reference layer of the optical disc and generating a reference detection signal, receiving a reflected information optical beam which is the information optical beam reflected by the mark layer of the optical disc and generating an information detection signal, generating a reflected reference beam intensity signal representing the intensity of the reflected reference optical beam on the basis of the reference detection signal and generating a focus error signal, which varies depending on a distance from the mark layer to the focal point of the detected optical beam in the proximity of the mark layer, on the basis of a plurality of information detection signals, and, with change in the reflected reference beam intensity signal as a momentum, starting focus control based on the focus error signal.